1. Field of the Invention
The present invention relates to a process for recycling side dams for the continuous casting of thin steel strip.
2. The Prior Art
Hexagonal boron nitride (BN) is customarily densified by hot pressing pulverulent BN to give bodies having a high sintered density. Side dams for the continuous casting of thin steel strip are made from these sintered bodies by sawing, turning or milling. Apart from pure BN, various mixed ceramics comprising BN and oxides, comprising BN and a mixture of oxides and carbides or comprising BN and sialons (silicas, aluminas, oxides, nitrides) are also produced by hot pressing. Such materials are described, for example, in EP 0 195 417, DE 38 24 849 A1 and U.S. Pat. No. 4,885,264. Customary compositions of such mixed ceramics are
a) for BN-oxide-carbide mixed ceramics:
30-85% by weight of BN
10-50% by weight of ZrO2, MgO
5-20% by weight of SiC, ZrC, TiC;
b) for BN-oxide mixed ceramics:
60-80% by weight of BN
20-40% by weight of ZrO2, MgO.
DE-A-10015849 describes compositions by means of which moisture absorption after sintering can be avoided by reaction of the B2O3 during the course of sintering. Such compositions are:
c) for BN-oxide-carbide mixed ceramic:
21-83.8% by weight of BN
10-50% by weight of ZrO2, MgO
5-20% by weight of SiC, ZrC, TiC
1.2-9% by weight of an additive selected from the group consisting of oxides, carbides and nitrides of the elements Si, Al, Ti and reaction products of the specified additives with B2O3,
d) for BN-oxide mixed ceramic:
51-78.8% by weight of BN
20-40% by weight of ZrO2, MgO
1.2-9% by weight of an additive selected from the group consisting of oxides, carbides and nitrides of the elements Si, Al, Ti and reaction products of the specified additives with B2O3.
All of the above prior art documents and each of the above compositions are herewith incorporated by reference.
After use, the side dams for the continuous casting of thin steel strip typically display traces of mechanical wear and chemical wear at the points which have been in contact with the metallic melt. The life of the components is determined either by the mechanical and/or chemical wear or by plant or process parameters such as batch size of the melt to be cast or the casting conditions. At the end of the life of the component, it is typical for  greater than 90% of its volume not to have been eroded away either mechanically or chemically. Nevertheless, the entire components have hitherto been discarded and replaced by new components.
It is an object of the present invention to provide a process which allows environmentally friendly, low-waste reprocessing of side dams for the continuous casting of thin steel strip, hereinafter referred to as side dams, having a mechanically or chemically worn surface.
The above object is achieved according to the present invention by a process in which the mechanically or chemically worn surface of the side dams is cleaned by methods known per se and the side dams are subsequently converted into a BN-containing powder by crushing and milling.
The side dam is preferably cleaned by mechanical grinding of the wear surface of the side dam which has been in contact with a metal melt. This grinding removes the reaction layer which is formed during metallurgical use of the side dam. After the reaction layer has been ground away, the side dam has the same composition as an unused side dam.
A side dam which has been cleaned in this way is coarsely broken up by means of a jaw crusher and subsequently milled to a powder by means of a conventional milling apparatus, e.g. a ball mill, vibratory mill or impact plate mill.
The powder preferably has a particle size similar to a corresponding new powder which is used for the production of a side dam. This is generally a mean particle size of about 10-15 xcexcm.
The composition of the powder obtained in this way corresponds approximately to that of the powder mixture originally used. It is possible for merely a small increase in the oxygen content to occur during reprocessing according to the invention. However this can be greatly eliminated by appropriate reprocessing methods, e.g. milling under a protective gas atmosphere.
The milling in the process of the invention is therefore preferably carried out under a protective gas, preferably in a nitrogen gas atmosphere or in an argon gas atmosphere.
The invention therefore also provides BN powder mixtures which have been produced by the process of the invention. The recycled BN powder obtained in this way can be used as raw material for the production of new side dams. The recycled BN powder can be used in its pure form. Preferably the recycled BN powder is used with the addition of new fresh raw BN powder materials to form a BN powder mixture comprising recycled BN powder and the new fresh raw BN powder.
The term xe2x80x9cnew fresh raw BN powderxe2x80x9d means BN powder which has never before been previously used to make a side dam.
The proportion of recycled BN powder in the BN powder mixture is preferably in the range from 10% to 60% by weight, particularly preferably from 20% to 50% by weight, and very particularly preferably from 25% to 35% by weight. The percent by weight of the recycled BN powder is based upon the total weight of the BN powder mixture. The balance of the powder mixture up to 100% by weight is fresh new BN powder.
Astonishingly, it has been found that the side dams produced from recycled powder only or from using the recycled powder mixture have better properties than side dams which have been produced exclusively from fresh new powder raw materials.
Especially side dams containing 10-60% by weight of recycled BN powders, preferably 20-50% by weight of recycled BN powders, and particularly preferably 25-35% by weight of recycled powders, unexpectedly display improved properties such as higher density, greater flexural strength and greater hardness. The percent by weight of the recycled BN powder is based upon the total weight of the BN powder mixture. The balance of the BN powder mixture up to 100% by weight is new fresh raw BN powder.
The present invention therefore also relates to side dams for metallurgical applications which have been produced using a recycled powder according to the present invention.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying examples which disclose several embodiments of the present invention. It should be understood, however, that the examples are designed for the purpose of illustration only and not as a definition of the limits of the invention.